This invention relates to processes for preparing mannitol. More particularly, this invention relates to processes for preparing mannitol by catalytic isomerization of glucose to produce a mixture of glucose and mannose, followed by catalytic hydrogenation of the isomerizate.
It is well known that a mixture of sorbitol and mannitol in aqueous solution can be produced by catalytic hydrogenation of invert sugar, which is an approximately equimolar mixture of glucose and fructose. Invert sugar, in turn, is commonly obtained by inversion of sucrose. Processes for hydrogenation of invert sugar are disclosed, for example, in U.S. Pat. Nos. 2,759,024; 3,329,729; 3,705,199; and 3,763,246. The yield of mannitol is ordinarily about 24-26 percent by weight, based on total dry solids; this yield can be increased to about 30 percent by weight by appropriate choice of catalyst, as described in U.S. Pat. Nos. 3,705,199 and 3,736,246 cited supra. Mannitol is readily recovered from aqueous solutions containing both sorbitol and mannitol by fractional crystallization, as described for example in U.S. Pat. No. 3,632,656. Hydrogenation of invert sugar is an attractive commercial route to the production of mannitol when the price of sucrose (ordinary sugar) is low. However, recent sharp rises and fluctuations in the price of sucrose have indicated a need for alternate routes.
It is also known that glucose can be isomerized under alkaline conditions to yield a mixture containing glucose, fructose, and mannose. Various alkaline materials for this purpose, including sodium hydroxide and calcium hydroxide, are known. The proportions of glucose, fructose, and mannose in the reaction mixture will vary depending on the alkaline material and the conditions used, and significant quantities of mannose are not always obtained. References describing the alkaline isomerization of glucose include U.S. Pat. No. 3,256,270 and W. Pigman, "The Carbohydrates: Chemistry, Biochemistry, and Physiology", Academic Press, New York, 1957, pages 60-69.
It is also known that both D-glucose and D-mannose can be epimerized in an aqueous solution containing about 17% by weight of D-glucose or D-mannose, based on total solution weight (i.e., 20% by weight of glucose, based on the weight of water) and a small amount of molybdic acid catalyst producing a solution containing 25 percent D-mannose, remainder D-glucose on the dry basis. This epimerization is described by V. Bilik, Chem. Zvesti, 26, 183-186 (1972) and in Czech patent 149,051 (June 15, 1973). No reaction took place when sodium molybdate was used as the catalyst. Bilik and co-workers have also found that various other sugars can be epimerized in the presence of molybdate ions in aqueous solution. For example, D-fructose gives a mixture of D-sorbose, D-tagatose, and D-psicose, and sucrose gives a mixture of D-mannose and D-sorbose in a similar reaction. [Chemical Abstracts, 80, 146433s (1974)]. Also, L-arabinose gives a mixture of L-arabinose and L-ribose [Czechoslovakian Patent 149,472, July 15, 1973; C.A. 81, 78189k (1974)]; L-mannose gives a mixture of L-glucose and L-mannose [C.A. 81, 91887f (1974)]; and D-galactose gives a mixture of D-talose and D-gulose in addition to D-galactose [C.A. 81, 152523h (1974)].